DENIERS SAY:

You can’t rely on the global CO2 data. Ice cores are inaccurate, and modern instruments are often placed near volcanoes.

SCIENCE SAYS:

From Hawaii to Antarctica, data collected from all over the world tell us one thing: Carbon pollution is going up.

Scientists don’t just measure carbon dioxide in one way, or in one place. Gas bubbles trapped in ice cores show us what carbon dioxide concentrations were like deep in the Earth’s past. Measurements made at the Mauna Loa Observatory in Hawaii show us that carbon dioxide has been increasing in our atmosphere. And Mauna Loa isn't the only place this pattern is seen; data from observatories in Alaska, American Samoa and Antarctica all show the same upward trend. All the measurements we have from an extensive array of sources tell us this: Carbon dioxide levels are rising.

Additional info from Skeptical Science

The following graph shows atmospheric CO2 levels over the last 10,000 years. It includes ice core data for CO2 levels before 1950. For values after 1950, direct measurements from Mauna Loa, Hawaii were used.

Mauna Loa is often used as an example of rising carbon dioxide levels because it’s the longest continuous series of directly measured atmospheric CO2. The reason it's acceptable to use Mauna Loa as a proxy for global CO2 levels is because CO2 mixes well throughout the atmosphere. Consequently, the trend in Mauna Loa CO2 is statistically indistinguishable from the trend in global CO2 levels (Figure 2). If global CO2 was used in Figure 1 above, the resulting "hockey stick" shape would be identical.